Problem: In math class, the girl to boy ratio is $5$ to $4$. If there are a total of $18$ students, how many boys are there?
Solution: A ratio of $5$ girls to $4$ boys means that a set of $9$ students will have $5$ girls and $4$ boys. A class of $18$ students has $2$ sets of $9$ students. Because we know that there are $4$ boys in each set of $9$ students, the class must have $2$ groups of $4$ boys each. There is a total of $8$ boys in math class.